I just back from the Salvation Army where I bought a rowing machine and one of those stepper things. They were only around $30 US so I couldn't resist. My basic idea is just using them for cardio on the off days, but the rowing machine has pretty big potential for muscle building (I think) so I thought maybe sub a bit of rowing for pushups but I really have no idea.

Does anybody have some suggestions on how to incorporate these into the workout?

Are you familiar with the Tabata Protocol? It is a four minute workout that is supposed to be very intense.

In a nutshell, you, in this case, row at high intensity (>30SPM) , for 20 seconds, then rest for 10 seconds, 8 times. From what I've read, it is amazingly effective at increasing fitness levels. The protocol appears in some crossfit WODs. The protocol can be adapted to many other excercises.

I've only just started reading about the Tabata method and it sounds like it would be extremely challenging. I used to incorporate HIIT training into my schedule and it sounds similar. HIIT stands for High Intensity Interval Training and can be applied to most cardio activities. The way it works is that you perform the exercise at peak intensity for 30 seconds, then at a slightly lower level for 30 seconds and repeat. So for running, after you warm up, you would sprint for 30 seconds, then jog for 30 seconds, sprint for 30 seconds, jog for 30 seconds etc. until you complete the full time. If your cardio is the pits, like mine, you can alternate jogging and walking. I think HIIT would work pretty well with either a rowing machine or a stepper.

One last thing, quite a few studies have shown that a 4-minute HIIT session provides the same cardiovascular and fat-burning benefits as a 40 minute session at moderate intensity. Only 4 minutes for cardio?? Who wouldn't love that

Thanks for the replies guys. I've heard about Tabata and HIIT before but I haven't tried either yet, though I am doing light interval training on my runs using the couch to 5k plan. I haven't picked the machines up yet so I'll have to stick with running until I do, but thanks for the links and ideas.

Here is a tabata variation put together by Cosgrove
The Routine:
Warm up for five minutes

Round: Perform 1 minute as fast as you can (a level 9 or 10 intensity \x96 on a scale of 1-10).
Recover at a moderate pace for two minutes (a level 6-7 intensity).
That's one "round" \x96 and it lasts three minutes
Cool down for five minutes

Weeks One to Four: Perform three rounds, three times per week.

The total cardio time will be 19 mins per workout including warm up
and cool down.

Weeks Five to Eight: Perform four rounds, four times per week.

The total cardio time will be 22 mins per workout including warm up
and cool down.

Weeks Nine to Twelve: Perform five rounds, four times per week.

The total cardio time will be 25 mins per workout including warm up
and cool down.

Weeks Thirteen to Sixteen: Perform six rounds, five times per week.

The total cardio time will be 28 mins per workout including warm up
and cool down.

Here is tabata ala crossfit!

Tabata Intervals ( 20 seconds of work followed by 10 seconds of rest repeated 8 times) is applied in turn to the Squat, Rower, Pullups, Sit-ups, and Push-ups with a one minute rotation break between exercises. Each exercise is scored by the weakest number of reps (calories on the rower) in each of the eight intervals. During the one minute rotation time allowed the clock is not stopped but kept running. The score is the total of the scores from the five stations.
Some performance insights and a scoring example from Mark Twight:

1. Lying down between exercises lowers HR faster than standing, sitting or walking, indicating better recovery in the short 60 second rest.
2. Alternating upright exercise (squat, pull-up) with prone or seated exercises produces lower heart rates, and allows greater overall level of work
3. Rowing first reduces reps on all other exercises
4. Rowing reps are not seriously affected if done last
5. Improvement happens really fast when the workout is done consistently (bimonthly).
6. High number of reps may be maintained for greater number of sets as fitness improves. Rep totals do not necessarily improve per set, but now I can do 6 sets of 7 pull-ups rather than doing 11, 8, 5, 4, 4, 4, 4, 4, etc. which suggests that local area endurance and lactic acid tolerance improve with this protocol.

Scoring Example:
A total score of 53 (Execllent score, BTW) is determined by adding up the lowest number of reps in any set of each exercise.
18 squats
4 pull-up
6 push-up
13 sit-up
12 row (use the calorie counter and call each calorie a rep)
This score is a 53.

I use Tabatas a lot for my training, and I disagree slightly with one of Mr. Twight's points. When you do exercises you'll experience either general exhuastion (call it whatever you like - no particular muscle group is toast, but you're breathing very hard, heart rate through the roof, etc.) or exhaustion in one muscle group (try doing Tabata pushups - you'll probably have serious pain in the arms/ chest/ shoulders, but you won't have the same general cardiovascular stress as when you do Tabata power cleans).
If you're experiencing general stress, by all means lie down. But when I do, for exampe, some squat type exercises, my thighs are really burning at the end of each set, and I prefer to walk slowly - I feel it helps flush metabolic products out of the muscle in question faster than lying down. Obviously, this is an easy experiment to do yourself.
For other exercises you could see my blog. I'm partial to one arm snatches with a dumbell, alternating jumping pullups/ air squats, or doing martial arts techniques with dumbells in hand.
By the way, HIIT is the generic term for these protocols, and Tabatas are just the specific ratio used by that researcher (i.e. 20 seconds work/ 10 seconds rest). I would think that if you prefer slightly longer work sets or rest it wouldn't be horribly different.
Any HIIT protocol will probably do you a lot more good than just climbing into your rower and pulling at a steady pace for 25 minutes, which is what most people probably do.

I just came back from the Salvation Army again, and I bought another indoor rower. This time it's one with a flywheel instead of pneumatic resistance. The one I bought before was in pretty bad shape and made the most irritating screeching noises when I used it, so I'm glad I found this one.

The best thing? It cost the same as the old one, i.e. almost nothing for a machine that would have costed a few hundred bucks new. Oh, and I also bought an ab wheel. That thing kills.

I've been doing some Tabata on and off for a while now, and it's really amazing how you feel after only 5 minutes as compared to doing 30 minutes normally.